Demonstrators originally obtained a march permit from the city for May 19, the day of the G8 summit.
But the Obama administration moved the G8 from Chicago to Camp David. So protesters amended their permit request, requesting their march take place on May 20, the start of the NATO summit.
Thursday, a judge backed the city's claim, that a march through the heart of the Loop would clog traffic and drain police resources.
It is a move protest organizers say is an attempt to block their 1st Amendment rights.
"We are going to be marching on Sunday, March 20, I can tell you that," said march organizer Andy Thayer. ""Just what form that will take-- that's why we have democratic decision-making."
Thayer said their next move could be taking this battle to a higher court.